For the Zaurus a special kernel+initramfs has been developed.
This special-purpose kernel is small enough to be flashed on NAND and features a framebuffer graphical menu for the selection of boot media containing the kernel to be kexec'ed ub the second phase.

Multi-partition cards and lots of filesystems are supported.
The kernels to be launched are searched for in /boot of each available partition.
There is a also a configuration file (/boot/boot.cfg) which allows selection of other kernels, custom kernel command lines, and specifies the label and the icon for each instance.

How to install:

Just unpack zaurus-installer-YOURMACHINE.tar.gz on a free (FAT or EXT2/3 formatted) partition: the package contains the kexecboot-kernel and the updater.sh utility. Then proceed as usual for flashing (switch off, pull battery out, insert battery again, before inserting AC plug press OK+Power On. In the Japanese Menu select 4 and finally 3 for SD card or 4 for CF).
Screenshot

For the SL-5500 (collie) and optionally for other Zaurus too, flashing is possible just using the routines of the original firmware:
reset the unit, Press C and D button during restart.
See: Flashing-collie

NOTE:

If you want a rootfs in nand, this must be in the .jffs2 format. Just rename your-image-rootfs.jffs2 to initrd.bin and copy it on the card with updater.sh. The image will be flashed on mtd2.